Three-terminal capacitive apparatus for remotely responding to a condition or dielectric properties of a material

ABSTRACT

Three-terminal capacitive apparatus are disclosed which provide information concerning a condition of a material by responding to the dielectric properties of the material. A three-terminal capacitor, which may be a cell or probe, may be used, for example, to measure the dielectric constant of a material, to determine the level of a material in a container, to determine the interface between liquid materials in a pipeline, or the proximity of a material to the probe. Each cell or probe includes a conductive driven element connected to a regulated square wave source, and a conductive receptor element connected to the input of a high gain amplifier with a capacitor in the feedback loop which maintains the receptor element at virtual ground. The output of the feedback amplifier is proportional to the feed through capacitance of the capacitive probe or cell. A synchronous demodulator, synchronized by the drive signal or a signal in phase with it, is connected to the feedback amplifier output and is used to produce a DC output signal used for display or control. The cell or probe is connected to associated electronics through shielded cables and may be remotely located from the electronics. Suitable mechanical apparatus is associated with a particular three-terminal capacitor cell or probe for mounting it in position to respond to a particular condition being monitored.

United States Patent 1191 Hardway, Jr.

11 1 3,77%},238 1451 Nov. 20, 1973 MATERIAL [75] Inventor: Edward V.Hardway, J12, Houston,

Tex.

[73] Assignee: Spearhead, Inc., Houston, Tex.

[22] Filed: Dec. 9, 1971 [21] Appl. No.: 206,463

Primary Examiner-Stanley T. Krawczewicz Attorney-W. F. Hyer et 21.

[57] ABSTRACT Three-terminal capacitive apparatus are disclosed whichprovide information concerning a condition of a material by respondingto the dielectric properties of the material. A three-terminalcapacitor, which may be a cell or probe, may be used, for example, tomeasure the dielectric constant of a material, to determine the level ofa material in a container, to determine the interface between liquidmaterials in a pipeline, or the [52] ELS. Cl. 324/61 R, 73/304 C,137/392, proximity 0f a material to the probe Each ll or 340/258 C,340/244 C probe includes a conductive driven element connected [51]lint. 1C1 6011' 27/26 to a regulated Square wave source, and aConductive [58] Field of Search 324/61 R, 61 P; receptor elementconnected to the input f a high gain 73/304, C, 398 C; 340/258 244 C;amplifier with a capacitor in the feedback loop which 137/392 386maintains the receptor element at virtual ground. The output of thefeedback amplifier is proportional to the [56] References Cted feedthrough capacitance of the capacitive probe or UNITED STATES PATENTScell. A synchronous demodulator, synchronized by the 3,519,923 7/1970Martin 324/61 R drive Signal or a signal in Phase with it, is connectedto 3,161,054 12/1964 Cohn 73/304 C the feedback amplifier output and isused to produce 3,375,716 4/1968 Hersch 73/304 C a DC output signal usedfor display or control. The 2,820,987 l/l958 Bunch 324/61 R cell orprobe is connected to associated electronics 3,486,103 12/1969 Roslca F324/61 R through shielded cables and may be remotely located g; 2 E fromthe electronics. Suitable mechanical apparatus is e (:0 e a. 3,611,12610 1971 Lucka 324/61 R i a .three Emma] capacitor 3,684,953 8/1972 Grant324/61 R Ce Pro Ff 'f 908mm to respo to 3,037,165 5 1962 Kerr 324/61 P aPam:ular condltlon bemg momt'ored- 14 Claims, 19 Drawing Figures REGULATED SQUARE R FERENCE WAVE 1 SOURCE *1 I f SYNCHRONOUS DEMODULATOR 50 :F-1 I L 23 I I L 1 G -e T PATENTEunuvzoms SHEET :3 CF 4 Pmmznnuvzm3,774,238

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SHEET a REFERENCE SQUARE WAVE DRIVE OUTLET Er azzzzm: m L p E THRESHOLDSOURCE DETECTOR REFERENCE I6 SQUARE WAVE DRIVE SYNCHRONOUS DEMODULATORALARM ---THRESHOLD OR CONTROL DETECTOR SIGNAL THREE-TERMINAL CAPACITIVEAPPARATUS FOR REMOTELY RESPONDING TO A CONDITION OR DIELECTRICPROPERTIES OF A MATERIAL This invention relates to three-terminalcapacitive apparatus for remote sensing or measuring applications, andin one of its aspects to three-terminal capacitor cells and probes andassociated electronic input and output circuits for use in suchapparatus, wherein the feed through capacitance of the cell or probe isa function of the dielectric properties of a material in proximity to orabout and between the capacitive elements of the cell or probe.

It is often desirable to use a capacitive device, such as a probe orcell, to determine the dielectric properties, and small changes therein,of a material in proximity to the cell or probe, and in the presence ofrelatively large cable capacitance and other stray capacitance. In manycases, it is either impractical or impossible to lo cate the measuringelectronic circuits at the point of measurement and remote sensing mustbe provided for by the apparatus employed.

Examples are in the measurement of level of noncon ducting liquids andgranular solids in a tank, and the determination of dielectric constantin on-stream process applications for the purpose of measuring orcontrolling the ingredient ratio or moisture content of the productbeing processed. In some instances, the detection of a small dielectricchange is important, such as when using high and low limit probes tocontrol the level of material in a storage tank or when detecting theappearance of an interface between two liqids flowing in a productspipeline. In the latter case, the dielectric changes and thus changes infeed through capacity of the probe may be extremely small compared tocable and other stray capacitances to ground. Another importantindustrial application for the detection of small changes in dielectricor conducting properties is that of proximity detection in which theapproach or presence of either a conducting object insulated fromground, or the approach or presence of a material of higher or lowerdielectric constant in the vicinity of or between the probe elements isdetected. Heretofore, in so far as is known to applicant, suitable andrelatively inexpensive devices for providing the desired measurement ordetection in remote sensing applications such as described have not beenavailable.

For example, in the past, four arm capacitance bridges and tunedresonant circuits with servomechanism balancing means in the indicatoror controller have been used in industrial devices for measuring level,moisture content, etc. These have been limited in accuracy and inapplication by the fact that they employed two-terminal capacitivemeasuring techniques wherein the capacitance of connecting cables andthe stray capacitance to ground were included in the measure ment. Thesecapacitances had to be subtracted by the apparatus or in theinterpretation of the result. Changes of the cable dielectric withtemperature would affect the result and no changes could be made inlength or type of cable furnished by the manufacture. Thus, theapplication to which the device was put was necessarily limited. Also,two-terminal devices which include cable capacitance with thecapacitance being sensed are far less suitable for remote sensingapplications, particularly where extremely small capacitive changes maybe involved.

Some analog and digital capacitance meters which are available measurethe current through or voltage across a low value resistance oreffective resistance in series with a capacitor connected to asinusoidal voltage source of precisely controlled amplitude andfrequency. However, these instruments develop amplitude and phase errorsaffecting their output in the presence of either cable capacitance orconductive leakage across the measured capacitor. Any error or changesin frequency directly affect the output.

Three-terminal automatic capacitance bridges which employ phasesensitive detectors to provide a polarized signal to drive a digitallycontrollled bridge have also been provided. These are insensitive tocable capacitance and shunt resistance or resistance to ground when atbalance. Typically, in these devices, a digitally switched variablevoltage multiplied by the value of the unknown capacity is balancedagainst a fixed voltage reference capacity product. Such bridges areused extensively for the measurement and sorting of capacitors. Theyare, however, extremely expensive and are unsuited to many industrialapplications because of their cost. These devices generally provide adigital indication and output but no analog output.

In utilizing capacitive devices in the industrial applicationspreviously mentioned, in some instances, leakage conductance in thecables or connections may be present. In other cases, the measuredmaterial may have some conductance. It is highly desirable to have themeasuring apparatus relatively insensitive to conductance between theconductors involved in the measurement, or between any one of them andground, and previous systems other than automatic bridges for thepurposes contemplated by the present invention have not provided anysimple and satisfactory means for rendering the measuring apparatusinsensitive to conductance.

It is thus an object of this invention to provide capacitive apparatusfor continuously measuring or responding to small changes in capacitancecaused by a condition of a material, and wherein such capacitivemeasurements can be made in the presence of cable and shunt capacitancewithout employing automatic capacitance bridges and phase sensitivedetectors.

Another object of this invention is to provide such apparatus which maybe used in numerous industrial applications in which the point ofmeasurement is remotely located from electronic apparatus employed.

Another object of this invention is to provide such apparatus whichprovides for the substantial cancellation of any shunt conductance ofthe measured material or conductance otherwise present in the measuringapparatus.

Another object of this invention is to provide such apparatus formeasuring level of a material in a container, or for determining whenthe material is above or below a certain point in the container toprovide a control or warning signal.

Another object of this invention is to provide such apparatus formeasuring dielectric constant of a material to determine moisturecontent and/or ingredients ratio.

Another object of this invention is to provide such apparatus fordetermining when the interface between two materials of differingdielectric constant passes by a certain point in a pipeline.

A further object of this invention is to provide such apparatus fordetermining the approach, presence, or proximity of a dielectric, orconducting object or material.

Another object is to provide such apparatus which can accomplish theabove objects while utilizing relatively simple and inexpensive circuitcomponents.

These and other objects of this invention, which will become apparentupon consideration of the descriptions herein and appended claims anddrawings are accomplished according to this invention by providingcapacitive apparatus including a three-terminal capacitor, which may bea cell or probe, mechanically arranged and mounted to perform aparticular function (as hereinafter described in detail). Thethree-terminal capacitor cell or probe includes a conductive drivenelement, a conductive receptor element, and a ground or guard terminalshielding the drive and receptor circuits from each other except in thearea of interest in the vicinity of or between the capacitor elements. Adrive signal source provides a square wave drive signal of closelyregulated amplitude to the driven element of the three-terminal cell orprobe. The input of a high gain amplifier with a capacitor in a feedbackloop is connected to the receptor element through a shielded cable withthe latters shield connected to ground or clamped at virtual ground by aunity gain amplifier. The feedback amplifier with a capacitor in thefeedback loop provides an output signal proportional to the feed throughcapacitance of the cell or probe and maintains the receptor element atvirtual ground potential by acting as a very large effective shuntcapacitance. The square wave drive source has a low output imped ance tomake it substantially insensitive to cable length. Thus, both the driveand output circuits may be connected to their respective terminals ofthe cell or probe by coaxial or shielded cables of varying length, andvery small changes in the feed through capacitance of the cell or probecan be detected.

The amplified output of the feedback amplifier is connected to a phasesensitive detector, (or synchronous demodulator) which is also connectedto the drive circuit so that it has a reference switching signalsubstantially in phase with the drive signal. The demodulator provides apolarized direct current output signal which may be amplified or feddirectly to an analog or digital indicator, high or low limit detectors,control circuits, etc., depending on the application to which theapparatus of this invention is to be put. The synchronous demodulatornot only cancels random noise or signals of other frequencies, but alsosubstantially cancels the effect of conductance between the capacitorelements of the cell or probe.

Various modifications of the three-terminal capacitor described can bemade to adapt it to different applications, a number of which aredescribed in detail below. In all such applications, the proximity of aconducting material or a dielectric material to the threeterminal probeor cell causes what may be but a small change in the feed throughcapacity of the cell or probe, and this change can be accuratelymeasured even in the presence of large cable or other stray capacitancesand with connecting cables of varying length so that the probe may beremotely located from the input and output electronics. Also, in allinstances, at least three terminal connections are used, i-.e., the

connection to the driven element, the connection to the receptorelement, and the ground or guard connection, and in all instances, onlythe feed through capaci tance between the driven and receptor elementsof the cell or probe is measured. The driven and receptor elements andthe circuits connected to them are completely guarded or shielded fromeach other except in the area of the measurement.

In the drawings, wherein like reference numerals are used throughout torepresent like parts, and wherein is illustrated preferred embodimentsof this invention:

FIG. I is a schematic diagram of the three-terminal capacitive apparatusof this invention showing the equivalent circuit of the feed throughcapacitance, var ious stray capacitance, and conductance;

FIGS. 2A, 2B and 2C show the wave forms of the drive signal and outputsignals in the FIG. I apparatus without the presence of shuntconductance across the capacitor elements;

FIGS. 3A, 3B and 3C show the same wave forms as FIG. but with thepresence of such conductance;

FIG. 4.- shows a tank partially filled with a nonconducting liquid witha three-terminal capacitor level probe installed therein;

FIG. 5 is an enlarged view of the probe in FIG. 4;

FIG. 6 is an end view of one of the nonconducting separators used tomaintain the spacing between the driven and receptor elements of theprobe in FIGS. 4 and 5;

FIG. '7 is a cross-sectional view of a three-terminal capacitor probefor use in measuring the dielectric constant of a material and indetecting sudden changes in dielectric constant of products flowing in apipeline;

FIGS. 8A and 8B are end views of the probe in FIG.

FIG. 9 is a schematic diagram of an interface detector for use with theprobe shown in FIGS. 7 and 8;

FIG. It) is a wave form diagram relating to the apparatus of FIG. 9;

FIG. Ii shows a three-terminal capacitive level control system formaintaining the level of material in a tank;

FIG. I2 is a schematic diagram of a three-terminal capacitor probe usedas a proximity detector; and

FIGS. 113A and H313 are end views of the probe of FIG. I2.

Referring to the drawings, in FIG. I, a three-terminal capacitor C ofcapacitance C,, which may be a cell or probe is illustrated as mountedin a shielded and grounded housing It). For example, housing It) may bea dielectric cell filled with a material M (shown by dots) the moisturecontent of which is to be determined. Capacitor C includes a drivenelement 11 com nected to a drive terminal 12, and a receptor element 13connected to a receptor terminal 14. The third terminal ofthree-terminal capacitor C is circuit ground. A drive source I5, whichprovides a regulated square wave drive signal E at its output, isconnected at its output by a shielded cable 16 to drive terminal 12 andthus to drive element 11. Receptor terminal 14 (and thus receptorelement 113) is connected by a shielded cable iii to an output circuitI9 which includes a high gain amplifier 2t feedback capacitor C ofcapacitance C, and a synchronous demodulator 21. The input of amplifier2t which preferably has a very high gain in the order of SJIINI orgreater, is connected to terminal M and to a feedback capacitor Cconnected to the output of amplifier 20,. This arrangement of amplifier20 insures that the signal level at terminal 14 will be maintained at orclamped to a very low signal level with respect to the drive signal Eand preferably at substantially zero signal level, so that the signallevel of receptor element 13 is, in effect, at virtual ground. Theoutput of amplifier 20 is connected to an input of synchronousdemodulator 21 and another input of demodulator 21 is connected to theoutput of drive circuit 15. In this manner, when drive signals E arapplied to terminal 12, current signals proportional to the feed throughcapacity of capacitor C, appear at terminal 14, which is clamped atvirtual ground potential by feedback amplifier 20. The amplifier outputvoltage, E,, of amplifier 20 is equal to the drive voltage E multipliedby the capacitance ratio C,/C,. The voltage E, is converted to apolarized direct current output voltage E at the output of synchronousdemodulator 21 synchronized by the drive signal E The output ofsynchronous demodulator 21 can be connected to a scaling potentiometer22 and to an appropriate analog or digital dis play or control device,such as the digital display 23 illustrated in FIG. 1..

In actual use, as contemplated by this invention, the elements of probeor cell in housing will be mounted so that the feed through capacitance,C,, between them is affected by the dielectric properties or conductiveproperties of a material, a condition of which is to be measured orsensed. Several stray values of capacitance and conductance which wouldnormally affect the accuracy of such determinations are shown in FIG. 1.G is the conductance in shunt with the capacitor C, and would normallyrepresent the conductance of the dielectric about and between capacitorC,. C and G represent capacitance and any conductance between receptorterminal 14 to ground including leakage in grounded shield cable 118. Cand G represent leakage conductance and capacitance from drive terminal12 to ground or to the grounded shield of cable l6. When long cables l6and 118 are used C and C are made up principally of capacitance in theconnecting cables. It should be noted that the reference voltageoriginating in the square wave drive E is completely shielded from thelow terminal circuit and no capacitance exists between these two exceptthe capacitance between the capacitor elements 11 and 13 in groundedhousing 110.

In the embodiment shown inv FIG. 1, the regulated square wave drive Emay typically be a ten lcilocycle square wave clipped or regulated byzener diodes to have a preciselydetermined amplitude. In thisembodiment, frequency is not critical. It is important that the outputimpedance of the regulated square wave drive by very low in comparisonwith the impedance of the conductance G in parallel with the capacitanceC at the test frequency. The output impedance might typically be 25 to50 ohms or less resistive. The high gain amplifier 20 with the capacitorC in the feedback loop provides a very large effective capacitance (Cbetween receptor terminal 14 (and thus receptor element 13) and ground.The value of this effective capacitance is KC, where K represents theopen loop gain of amplifier 20. If the value of C j is 1,000 picofaradsand the gain of amplifier 20 is, for example, 5,000 thenthe value of Cwill beb mic rofarads. This effective capacitance is large enough tomaintain receptor element 13 very close to ground potential or virtualground. The.

shunt impedance of C and G must be very large in comparison with theimpedance of the shunt capacitor C,.,,. Generally speaking, the value ofC which is equal to KC,, must be large compared with the capacitance CUsually, the latter will consists principally of cable capacitance. Ithas been found experimentally that an added capacitor C, between theterminal 14 and ground also reduces the effect of added cablecapacitance. It is believed that this is because the rise time and slewrate limitations of amplifier 20 have less effect when such a capacitorC, is present to store charge while the amplifier output is, in effect,catching up with the signal on terminal 14. In one test case with afeedback capacitor C of 1,000 picofarads and a shunt capacitor C, with avalue of 1,000 picofarads connected between terminal 114 and ground, itwas found possible to measure values of C, to 0.1 percent with orwithout added cable lengths of up to 30 ft., equivalent to a change in Cof 900 picofarads. This was accomplished using an operational amplifierwith a gain of 50,000, a band width of 3 megacycles, and a slew rate of10 volts per microsecond for amplifier 20.

Referring to FIG. 2, the wave form represented on line A is that of thesquare wave drive E The wave form represented on line B is the output Eof amplifier 20 in the absence of any shunt conductance G The wave formon line C represents the output E of synchronous demodulator 21. Thelittle blips or gaps shown are present because of the fact that the risetime and the fall time of amplifier 20 are not instantaneous. If theserise and fall times were instantaneous, and if synchronization wereperfect, these blips would not be present and the output of thedemodulator would be pure DC. The output signal E is reduced by the gapscaused by the limited slew rate and amplifier rise time. The rise timechanges proportionally causes no error with changes in amplitude. Theslew rate limitation of the amplifier 20 causes some nonlinearity. Thiseffect is reduced by lowering the test frequency. If, however, thefrequency is too low, conductance effects as shown in FIG. 3, and laterdescribed, will saturate amplifier 20. Since higher slew rate amplifiersare more costly, a compromise must be made to fit the applications.

In FIG. 3, on line A, the square wave drive E is identical to that shownin FIG. 2, line A. The wave form on line B of FIG. 3 is the same as thaton line B of FIG. 2 with the exception that it shows the presence of asignificant amount of shunt conductance G which simply is the reciprocalof the resistance appearing across C which can be denoted as R The waveform on line B in FIG. 3 is, again, the output of feedback amplifier 20designated as E, in FIG. I. The wave form shown on line C in FIG. 3 isthe output E of synchronous dentedulator 21 when its input is the waveform shown on line B. Some actual numbers may be useful explaining theoperation of the synchronous demodulator in the presence of some shuntresistance R across the capacitor C, such as when a dielectric materialsuch as M is about and between elements 11 and 13. If the drive voltageE shown is assumed to be 6 volts, zero to peak, the feedback capacitanceC, is 300 picofarads and the capacitance C, is picofarads, the outputvoltage E, of feedback amplifier 20 is given as follows:

From the above equation it can be seen that the out put voltage E, offeedback amplifier 20 will be two 'volts. -Th e sloping lines ofcurvesl3 and C in FIG.. 3 are due to the integrating action of feedbackamplifier 20 in the presence of resistance connecting it with a squarewave source. The rate of change of voltage represented by the slopinglines or ramps with respect to time is simply the drive voltage Edivided by the time constant R C In the illustration shown, each rampmoves through a change of 6 volts in about 0.5 X 10 seconds. The shuntresistance can, therefore, be computed to be 167,000 ohms.

The remaining FIGS. of the drawings illustrate various applications ofthe apparatus and principles described with respect to FIGS. I3 and inwhich capacitor C is replaced by a cell or probe and shunt conductanceG, includes conductance caused by the proximity of a material beingmeasured to the cell or probe. For example, capacitor C, may be athree-terminal cell which is filled with a grain for determination ofthe moisture content, and the dielectric constant of the grain (which isa function of its moisture content) and conductance of the grain willaffect the feed through capacitance C, of the cell in the mannerdescribed.

FIG. 4 shows a tank 24 containing a liquid L and a probe 25 comprised oftwo long tubes or rods 26 and 27 extending from the top center of thetank to near the bottom of the tank. The two rods 26 and 27 mayrepresent the two elements 11 and 13 of a three-terminal capacitor C,-such as illustrated in FIG. 1, and the capacitance of probe 25 can bedesignated as C). Any conductance of the liquid between the probeswould, again, correspond to 6,. With probe 25 connected into the circuitof FIG. 1, as the liquid in the tank rises the capacitive couplingbetween rods 26 and 27 will increase causing an increase in outputsignal E from demodulator 21.

FIG. is an enlarged view of a preferred form of probe shown in FIG. 4.Rods 26 and 27 are insulated from each other in a spaced apartrelationship by plastic insulators 28 (see FIG. 6) of which five areillustrated in FIG. 5. Connections to the driven and receptor elementsof the probe are made through shielded cablesin a circuit similar tothat shown in FIG. 1, so that if rod 26 represents driven element 11, itwould be connected to cable 16, and if rod 27 represents receptor 13, itwould be connected to cable 18.

As illustrated in FIG. 5, each of rods 26 and 27 is mounted in aninsulated block 29 which is in turn mounted in a shielded housing 30comprising upper and lower shielding shrouds 30a and 30b, and alaterally extending flange member 31. Flange member 31 can be mounted bysuitable bolts on the top of tank 24 and over an opening through whichrods 26 and 27 can extend into the tank. The grounded shields on cables16 and 18 are connected to a shielding partition 32 mounted in housing30 to divide it into two shielded sections 33 and 34. Partition 32 isconnected to shroud 30a and grounded along with shroud 30b, flange 31and tank 24 so that rods 26 and 27 and the terminals and wiresconnecting them are completely shielded from each other except betweenthe portions of rods 26 and 27 extending into tank 24 for contact withliquid L.

FIGS. 7 and 8 illustrate a preferred form of threeterminal probe 35which may be inserted into a pipeline or dielectric constant cell byscrewing it into suitable threaded opening. Probe 35 may also beconnected in the circuit of FIG. 1 so that it would be represented bycapacitance C, of capacitor C in FIG. 1. Probe 35 includes twocapacitive elements or plates 36 and 37, and if plate 36 representsdriven element 11 of capacitor C then it would be connected by cable 16to drive source 15. Also, if plate 37 represents receptor element 13,then it would be connected to amplifier 20 by cable 18. Plates 36 and 37are respectively connected to rods 38 and 39 extending through a plasticinsulated mounting member 40. Plastic insulating member 40 is split inhalf by a conducting shield partition 41 to minimize all unwantedcapacitance between the driven and receptor elements 36 and 37, exceptin the area of interest, and between connecting rods 38 and 39 and theterminals connected to cables 16 and 18. insulating member 40 iscylindrical and is mounted in a cylincrical, externally threadedconductive plug 42. A conductive shielded shround 43 is mounted on topof plug 42 and is connected to partition 41 and to the grounded shieldof cables 16 and 18, and a conductive shield plate 44 is mounted on thebottom of plug 42 but insulated from rods 38 and 39. Plug 41 is alsogrounded through its connection with shroud 43 and shield plate 44 isthus grounded. When probe 35 is mounted in a cell or pipeline, plates 36and 38 are surrounded by' a material whose dielectric constants,moisture content, or ingredients ratio is being determined, and, asnoted, probe 35 may be used with circuits such as shown in FIG. 1 tomake measurements of dielectric constant, moisture content, oringredient mixture ratio of known ingredients of either liquid and solidmaterials.

FIG. 9 is a schematic diagram of a circuit similar to that shown in FIG.1 (with capacitor C replaced by a probe such as probe 35 of FIG. 7)wherein the output of synchronous demodulator 21 is coupled through acapacitor C to two adjustable potentiometers 45 and 46. In this and thesubsequent views, the grounded shield of the connecting cables arerepresented by the circle connected to ground about the linerepresenting the center conductor of the cable. The output ofpotentiometer 45 is connected to a positive threshold detector 47 andthe output of potentiometer 46 is connected to a negative thresholddetector 48. Each of these threshold detectors produces an output signalwhen the input signal to them exceeds a certain threshold value of theappropriate polarity. Sudden positive or negative changes of the DClevel of output signal E coming out of synchronous demodulator 21 willthus be differentiated by capacitor C to provide a positive or negativevoltage pulse and when the peak voltage of this pulse exceeds thethreshold value of detectors 47 and 48, depending on the polarity, oneof detectors 47 and 48 will produce an output signal. In thisapplication, probe 35, for example, may be screwed into a pipeline Fcontaining products F (illustrated by dots) which change from time totime. Petroleum product pipelines, for example, may pass gasoline forseveral hours and then switch to kerosene. It is quite important thatthe operator be able to detect the interface between the gasoline andkerosene the moment that it appears to prevent mixing. Althoughdielectric constants do change markedly with temperature, it isgenerally true that any known material being transmitted will haveeither a greater or a smaller dielectric constant than any othermaterial which is following, even though the difference may be quitesmall. A properly adjusted and sensitive positive or negative thresholddetector 47 or 48, therefore, can serve to detect the appearance of theinterface even though the change in dielectric constant is small andoccurs suddenly. FIG. 10 shows a wave form 49 such as would appear atthe input of the positive threshold detector shown in MG. 9 when thedielectric constant of the material in a pipeline suddenly changes. Whenthe transient shown on wave form 49 exceeds the threshold set level 50,it causes the level detector 47 (since it is positive) to be tripped,giving notice that the 1 interface has arrived at probe 35.

FIG. lll illustrates a capacitive level control system for maintainingthe level of material in a tank 51 at any desired level using probe 25previously described and shown in FIG. 5. When the material in tank 51rises to a point that increases the capacitive coupling between thedriven and receptor elements 26 and 27 of the probe 25, it will cause anincrease in the signal E, going into synchronous demodulator 211 andthus signal E at the output of demodulator 21. A threshold detector 52is connected to the output of synchronous demodulator 21 throughthreshold or level setting potentiometer P, and the coil of a relay 53is connected to the output of detector 52. Threshold detector 52responds to output signal E when it exceeds a certain threshold levelcorresponding to a desired material level in the tank 511 to activate oropen the contacts of relay 53. The contacts of relay 53 are normallyclosed and are connected between a source of power (not shown) and anormally closed solenoid valve 54 connected to control the flow ofmaterial into tank 51. Thus, detector 52 responds to the output ofsynchronous demodulator 21 and provides a signal to operate relay 52causing its contacts to open and causing solenoid valve 54 to close whenthe output E of demodulator 21 exceeds a certain level corresponding towhen the level of material in tank 511 is at or near a desired level.When the material in tank 51 falls below this level, the feed throughcapacitive coupling of probe 25 will decrease, decreasing the outputsignal from demodulator 21 and causing threshold detector 52 to stopproviding its output signal and allowing the contacts of relay 52 toclose. When these relay contacts close, solenoid valve 54 opens allowingadditional material to enter tank 51 and the process continues. Thelevel setting potentiometer P is used to set the desired level in thetank. A different scale would be required for materials of differentdielectric constant. The apparatus shown in FIG. 11 can be designed towork with materials which are very light and fluffy and have dielectricconstants very close to that of air. With circuitry disclosed herein, itis possible to detect changes and feed through capacity of the probe assmall as 0.001 picofarads in the presence of 1,000 or more picofarads ofcable capacitance to ground.

A proximity detector is shown in FIG. 12 as including a probe 55 whichcan be connected in the circuit of FIG. ll andsubstituted for capacitorC in FIG. 1. As in capacitor C of FIG. 1, probe 55 includes a drivenelement 56 connected by cable lid to square wave drive circuit l5, and areceptor element 57 connected by cable 18 to the input of integratingamplifier 20. The driven and receptor elements 56 and 57 in this casemay be thin strips of metallic material separated by a wider strip ddwhich serves as a shield and is grounded. These elements are molded intoa block 59 of nonconducting plastic material and the block itself isenclosed in a conductive metal housing 60 which is maintained at'groundpotential. When an ungrounded object 61 of conductive material or anymaterial with a dielectric tor probe 55, there is an increase incoupling between driven element 56 and receptor element 57. Theincreased coupling causes an increase in output of synchronousdemodulator 211. The output of synchronous demodulator 211 is connectedto the output of a threshold level detector 52 (such as described withrespect to 1? 1G. 1111) and the increased output 15 of demodulator 21trips threshold detector producing an alarm or control signal.

in this embodiment as well as that of FIG. ll, a potentiometer P isplaced between synchronous demodulator 211 and threshold detector 52 topermit an accu-- rate setting of the desired threshold. in both embodiments, the probes 25 and 5% may be connected through many feet of cableto their associated electronics.

The apparatus described in FllG. 112 is capable of readily sensingcapacitance changes of 0.00.] picofar ads in probe 55 in the presence ofcable capacitances to ground of 1,000 picofarads. Although thedielectric constant of cable insulation changes substantially withtemperature, in the above example, a change of only 1 percent would be10 picofarads and would have little effect on the accuracy of thecapacitance measurement made.

As used herein, the term stray capacitance broadly refers to the cableand the other stray capacitances between the capacitive element andground as noted with respect to the description of HG. T.

In all of the embodiments described, the use of a stable, preciselyregulated square wave drive signal makes possible both accuratemeasurement and the detection of very small changes in feed throughcapacitance of the three-terminal capacitor used. The affects of straycapacitance on the resulting output signal is effectively eliminated andthe circuits described are insensitive to small changes in frequency. Ahigh gain amplifier with an adequately large feedback capacitor is usedto clamp the receptor element of the three-terminal capacitor and allconductors connecting to it to virtual ground. The higher the frequencyof operation, the less likely is the circuit to be saturated from theeffect of conductance between the driven and receptor elements. Thepractical upper frequency limit is determined by several factorsincluding the output impedance of the square wave source, the gain, therise time and the slew rate of the amplifier and the amplifiers outputcurrent capability. The higher the gain of the amplifier and the largerthe feedback capacitor across it, the greater the tolerance of the:circuit to cable capacitance and changes in cable capacitance. Theeffect of conductance between the driven and receptor elements iscancelled in the synchronous demodulator. Utilization of thecombinations described make possible the application of capacitivetechniques to many industrial applications which were heretoforeimpractical for the reasons previously discussed.

In certain applications, such as in level measurement and control, itmay be desirable to form the capacitor C of parallel plates orconcentric cylinders immersed in material M to reduce the effect ofchanging dielectric constant with temperature, i.e., to make the ratioCJC, insensitive to temperature.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the apparatus.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed ll. Three-terminalcapacitive apparatus responsive to a condition of a material whichcauses changes in the feed through capacitance of the apparatusJso thatsuch changes may be relatively accurately sensed or measured even in thepresence of substantial amounts of stray capacitance, comprising, incombination: an..electrically driven conductive element; a conductivereceptor element; support means for supporting said driven andreceptorelements spaced from and adjacent to each other and in the proximity ofsuch material so that said condition affects the feed throughcapacitance of said capacitive apparatus; a source of square wave drivesignals having a substantially constant peak amplitude with respect tocircuit ground; first shielded conductive means connecting said sourceto said driven element to conduct said drive signals thereto, the outputimpedance of said source being such that any stray capacitance betweensaid driven element and ground is rendered substantially ineffectivewith respect to said amplitude of said drive signals; a high gainamplifier including a capacitive feedback loop for maintaining saidreceptor element at virtual ground while providing an electrical outputsignal responsive to the feed through capacitance of said capacitiveapparatus, and second shielded conductive means connecting said receptorelement to said amplifier.

2. The apparatus of claim 1 wherein the presence of conductance of thematerial in proximity to said driven and receptor elements detrimentallyaffects the accuracy of determination of the feed through capacitance ofsaid apparatus, and further including a synchronous demodulatorconnected to said amplifier and to said source of drive signals fordemodulating the output signals from said amplifier in synchronism withsaid drive signals to substantially cancel out the effects of any suchconductance present in said output signals.

3. The apparatus of claim 1 wherein the condition of said material isthe level of such a material in a container and said driven and receptorelements comprise a level probe supported by said support means in sucha container so that the feed through capacitance of the probe isresponsive to a level of the material in the container. I

4. The apparatus of claim 3 wherein said level probe is mounted in thetop ofa container and said driven and receptor elements extend from thetop of the container to adjacent the bottom thereof whereby said feedthrough capacitance is responsive to the level of the material in thecontainer throughout substantially all of 3 the container.

5. The apparatus of claim 4 further including a threshold detectorconnected to said amplifier and providing an output signal when theoutput of said amplifier exceeds a predetermined level, valve means'said material is the level of such a material in a container and saiddriven and receptor elements comprise a level probe supported by saidsupport means in such a container so that the feed through capacitanceof the probe is responsive to a level of the material in the container.

8. The apparatus of claim i wherein saidcondition of said material isits proximity to a particular point, and wherein said driven andreceptor elements comprise a probe supported by said support means-atsaid point, wherein the approach of said material to said point producesdetectable changes in the feed through capaci tance of said probe, andfurther including means connected to said amplifier and responsive towhen such changes in feed through capacitance exceed a predeterminedamount to provide a control signal indicating the proximity of saidmaterial to said point.

9. The apparatus of claim 2 wherein said condition of said material isits proximity to a particular point, and wherein said driven andreceptor elements comprise a probe supported by said support means atsaid point, wherein the approach of said material to said point producesdetectable changes in the feed through capacitance of said probe,'andfurther including means connected to said amplifier and responsive towhen such changes in feed through capacitance exceed a predeterminedamount to provide a control signal indicating the proximity of saidmaterial to said point.

10. The apparatus of claim 1 wherein said condition is the change indielectric constant of said material, and further including means forproviding an output signal in response to the rate of said change.

ll. The apparatus of claim 10 wherein said last mentioned means includesat least one threshold detector responding to said ,output signal whenit exceeds a predetermined level to provide a control signal, and acapacitor connected between the output of said output circuit and saidthreshold detector.

12. The apparatus of claim 2 wherein said condition is the change indielectric constant of said material, and further including means forproviding an output signal in response to the rate of said change.

13. The apparatus of claim 1 wherein said support means supports saiddriven and receptor elements in a dielectric cell and the condition ofsaid material is the dielectric constant of said material which isproportional to the moisture content or ingredients ratio of thematerial.

14. The apparatus of claim 2 wherein said support means supports saiddriven and receptor elements in a dielectric cell and the condition ofsaid material is the dielectric constant of said material which isproportional to the moisture content or ingredients ratio of thematerial.

1. Three-terminal capacitive apparatus responsive to a condition of amaterial which causes changes in the feed through capacitance of theapparatus, so that such changes may be relatively accurately sensed ormeasured even in the presence of substantial amounts of straycapacitance, comprising, in combination: an electrically drivenconductive element; a conductive receptor element; support means forsupporting said driven and receptor elements spaced from and adjacent toeach other and in the proximity of such material so that said conditionaffects the feed through capacitance of said capacitive apparatus; asource of square wave drive signals having a substantially constant peakamplitude with respect to circuit ground; first shielded conductivemeans connecting said source to said driven element to conduct saiddrive signals thereto, the output impedance of said source being suchthat any stray capacitance between said driven element and ground isrendered substantially ineffective with respect to said amplitude ofsaid drive signals; a high gain amplifier including a capacitivefeedback loop for maintaining said receptor element at virtual groundwhile providing an electrical output signal responsive to the feedthrough capacitance of said capacitive apparatus, and second shieldedconductive means connecting said receptor element to said amplifier. 2.The apparatus of cLaim 1 wherein the presence of conductance of thematerial in proximity to said driven and receptor elements detrimentallyaffects the accuracy of determination of the feed through capacitance ofsaid apparatus, and further including a synchronous demodulatorconnected to said amplifier and to said source of drive signals fordemodulating the output signals from said amplifier in synchronism withsaid drive signals to substantially cancel out the effects of any suchconductance present in said output signals.
 3. The apparatus of claim 1wherein the condition of said material is the level of such a materialin a container and said driven and receptor elements comprise a levelprobe supported by said support means in such a container so that thefeed through capacitance of the probe is responsive to a level of thematerial in the container.
 4. The apparatus of claim 3 wherein saidlevel probe is mounted in the top of a container and said driven andreceptor elements extend from the top of the container to adjacent thebottom thereof whereby said feed through capacitance is responsive tothe level of the material in the container throughout substantially allof the container.
 5. The apparatus of claim 4 further including athreshold detector connected to said amplifier and providing an outputsignal when the output of said amplifier exceeds a predetermined level,valve means mounted in said container for permitting and stopping theflow of material into the container, and switch means connected to theoutput of said threshold detector and to said valve means and responsiveto the output signal from said threshold detector to actuate said valvemeans.
 6. The apparatus of claim 5 wherein said threshold detectorincludes means for varying said predetermined level.
 7. The apparatus ofclaim 2 wherein the condition of said material is the level of such amaterial in a container and said driven and receptor elements comprise alevel probe supported by said support means in such a container so thatthe feed through capacitance of the probe is responsive to a level ofthe material in the container.
 8. The apparatus of claim 1 wherein saidcondition of said material is its proximity to a particular point, andwherein said driven and receptor elements comprise a probe supported bysaid support means at said point, wherein the approach of said materialto said point produces detectable changes in the feed throughcapacitance of said probe, and further including means connected to saidamplifier and responsive to when such changes in feed throughcapacitance exceed a predetermined amount to provide a control signalindicating the proximity of said material to said point.
 9. Theapparatus of claim 2 wherein said condition of said material is itsproximity to a particular point, and wherein said driven and receptorelements comprise a probe supported by said support means at said point,wherein the approach of said material to said point produces detectablechanges in the feed through capacitance of said probe, and furtherincluding means connected to said amplifier and responsive to when suchchanges in feed through capacitance exceed a predetermined amount toprovide a control signal indicating the proximity of said material tosaid point.
 10. The apparatus of claim 1 wherein said condition is thechange in dielectric constant of said material, and further includingmeans for providing an output signal in response to the rate of saidchange.
 11. The apparatus of claim 10 wherein said last mentioned meansincludes at least one threshold detector responding to said outputsignal when it exceeds a predetermined level to provide a controlsignal, and a capacitor connected between the output of said outputcircuit and said threshold detector.
 12. The apparatus of claim 2wherein said condition is the change in dielectric constant of saidmaterial, and further including means for providing an output signal inresponse to the rate of said change.
 13. The apparatus of claim 1wherein said support means supports said driven and receptor elements ina dielectric cell and the condition of said material is the dielectricconstant of said material which is proportional to the moisture contentor ingredients ratio of the material.
 14. The apparatus of claim 2wherein said support means supports said driven and receptor elements ina dielectric cell and the condition of said material is the dielectricconstant of said material which is proportional to the moisture contentor ingredients ratio of the material.